In alkali chloride electrolysis to produce an alkali hydroxide and chlorine by electrolyzing an aqueous solution of an alkali chloride (such as sodium chloride, potassium chloride or lithium chloride), an ion exchange membrane having a layer made of a fluorinated polymer having carboxylic acid type functional groups on the cathode side and having a layer made of a fluorinated polymer having sulfonic acid type functional groups on the anode side, is used as a diaphragm.
Such an ion exchange membrane is produced, for example, by the following method.
A method for producing an ion exchange membrane having a layer containing a fluorinated polymer having carboxylic acid type functional groups and a layer containing a fluorinated polymer having sulfonic acid type functional groups, by immersing an ion exchange membrane precursor film having a precursor layer containing a fluorinated polymer having groups convertible to carboxylic acid type functional groups, and a precursor layer containing a fluorinated polymer having groups convertible to sulfonic acid type functional groups, in an alkaline aqueous solution comprising an alkali metal hydroxide, a water-soluble organic solvent and water, to subject the groups convertible to carboxylic acid type functional groups to hydrolysis treatment to convert them to carboxylic acid type functional groups, and to subject the groups convertible to sulfonic acid type functional groups to hydrolysis treatment to convert them to sulfonic acid type functional groups (see e.g. Patent Documents 1 and 2).